Air conditioning systems for the passenger compartments of motor vehicles are well known. In general, these systems comprise an inside heat exchanger (located within the passenger compartment) and an outside heat exchanger (located outside the passenger compartment). A pair of fluid passages connect the heat exchangers to allow the circulation of fluid through the heat exchangers. An expansion device is positioned in one of the fluid passages. A compressor and accumulator/dryer is positioned in the other fluid passage. When fluid is pumped by the compressor through the outside heat exchanger, the expansion device, the inside heat exchanger and the accumulator/dryer in succession, air passing through the inside heat exchanger is cooled as the air flows into the passenger compartment. When fluid is pumped in the reverse direction through the inside heat exchanger, the expansion device, the outside heat exchanger and the accumulator/dryer in succession, air passing through the inside heat exchanger is heated as the air flows into the passenger compartment. A reversing valve can be positioned in the other fluid passage to provide the required flow direction for the fluid.
In general, a pneumatic arrangement is used to control the displacement of the compressor, the pneumatic arrangement having an essentially fixed control set point. Such an arrangement has restricted control and does not provide for full control of compressor displacement throughout the range of possible operating conditions, especially during heating of the passenger compartment. Also, the use of compressor speed control is unsuitable in motor vehicles because the compressor is driven by the engine of the motor vehicle, and the engine speed is continually varying.